1. Field of the Invention
The present invention relates generally to charge storage devices with at least one electrode that is composed of multiple networks of nanowires.
2. Description of Related Art
Electrochemical capacitors (also known as supercapacitors or ultracapacitors) have been attracting numerous interests because they can instantaneously provide higher power density compared to batteries and higher energy density compared to the conventional dielectric capacitors. Such outstanding properties make them excellent candidates for applications in hybrid electric vehicles, computers, mobile electric devices and other technologies.
Generally, an electrochemical capacitor may be operated based on the electrochemical double-layer capacitance (EDLC) formed along an electrode/electrolyte interface, or a pseudocapacitance resulting from a fast reversible faradic process of redox-active materials (e.g., metal oxides and conductive polymers). For an EDLC-based capacitor, the rapid charge/discharge process provides the capacitor with a high power density, yet the energy density is limited by its effective double-layer area.
To date, a large number of high-surface-area materials, such as activated carbon, templated carbon, and carbon nanotubes (CNTs), have been extensively studied. Activated carbons, with surface areas from 1000 m2/g to 2500 m2/g, are the most commonly used materials, which may provide a capacitance up to 320 F/g at low potential scanning rate. However, the capacitance may drop dramatically at high scanning rates because of their tortuous pore structure and high microporosity. The templated carbons, on the other hand, exhibit uniform pore geometry and larger pore size; however, they did not show any exciting improvement in either energy or power performance. For comparison, multi-walled CNTs show capacitances up to 135 F/g and single-wall CNTs show capacitances up to 180 F/g, which are still low for an actual device application.
Compared with the EDLC-based capacitors, pseudocapacitors based on transition metal oxides or conducting polymers may provide much higher specific capacitances up to one thousand farads per gram of the active material. However, their actual applications are still limited by high cost, low operation voltage, or poor rate capability, mostly because of inefficient mass transport or of slow faradic redox kinetics. Specifically, such high electrical resistance can limit the practical thickness (smallest dimension) of oxide electrodes, as increased thickness leads to increased electrode resistance and reduced charge transport.
Hybrid capacitors have been fabricated that integrate both the electric double-layer capacitance and pseudocapacitance within a single electrode. For example, ruthenium oxide nanoparticles have been loaded onto activated carbon, composites of MnO2 nanoparticles have been loaded on templated carbon, and ruthenium oxide nanoparticles have been loaded on carboxylated CNTs. Thin layers (6 nm) of vanadium-oxide nanoparticles have been electrodeposited on CNTs. However such composite thin films with low oxide loading levels and/or low film thickness may not be suitable for practical applications.
Consequently, in spite of extensive research and effort, making supercapacitors with high energy and power density still remains challenging. Supercapacitors electrodes of the prior art have not provided the device performance (e.g., energy density, power density, cycling stability, operating voltage) and manufacturability required for many high-performance, commercial applications.